Method of operating an inverter by turning off the switching at a zero transition of the alternating output current

ABSTRACT

A method of operating an inverter for converting direct voltage into alternating voltage. The inverter has direct-voltage terminals and alternating-voltage terminals and a plurality of power switching elements that are clocked at high-frequency connected between the d.c. and a.c. terminals. The high-frequency clocking of the power switching elements of the inverter is switched off around a zero transition of the alternating current or the alternating voltage for a period which depends on the direct voltage present at the direct-voltage terminals of the inverter and/or the output power of the inverter. No current is generated during time intervals with a poor efficiency.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation, under 35 U.S.C. §120, of applicationSer. No. 12/244,276, filed Oct. 2, 2008, now U.S. Pat. No. 8,169,805 B2;the application also claims the priority, under 35 U.S.C. §119, ofGerman patent application DE 10 2007 050 450.2, filed Oct. 19, 2007 andGerman patent application DE 10 2007 058 633.9, filed Dec. 5, 2007; theprior applications are herewith incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for operating an inverter forconverting direct voltage into alternating voltage, havingdirect-voltage terminals and alternating-voltage terminals, betweenwhich a plurality of power switching elements clocked at high-frequencyare connected.

Inverters are used, for example, for feeding electrical energy into thepublic power system when only direct-voltage energy sources such asphotovoltaic installations, fuel cells or batteries are available.Regardless of their type of construction, all inverters regularly absorbmore energy than they deliver. It is an object of many developments ofinverters, therefore, to improve their efficiency.

European Patent EP 0 284 021 B1, corresponding to U.S. Pat. No.5,021,936, describes clock patterns for generating sinusoidal voltageand sinusoidal current for transformerless inverters. Depending on thedesign of their components, the efficiency of inverters of that type isbest within a certain load range. Below and above that load range, theefficiency distinctly drops off.

German Published, Non-Prosecuted Patent Application DE 102 21 592 A1discloses an inverter in which two additional switches each having oneseries-connected diode, are used at the output of an inverter bridge.When those switches are opened and closed at the correct time, feedingenergy back from the chokes into the buffer capacitor during thecommutation of the power switching elements, is avoided. The ripple ofthe current in the chokes, and thus their losses, become less. However,that circuit configuration requires a large expenditure of componentsand scarcely improves the efficiency for feeding little energy into thesystem.

European Patent Application EP 1 626 494 A2, corresponding to U.S. Pat.No. 7,411,802 B2, also has the object of avoiding feeding energy backinto the buffer capacitor during the commutation of the power switchingelements of the inverter. That is achieved by an additional switchbetween the buffer capacitor and the inverter bridge which interruptsthe line to the buffer capacitor during the commutation. In theremaining time, however, that additional switch must conduct currentand, as a result, itself again causes a loss of power. In addition, theefficiency of the inverter is scarcely improved when little energy isfed into the system.

Photovoltaic installations supply fluctuating power to the system inaccordance with the fluctuating solar irradiation due to cloud coverageof the sun and diurnal variation. An inverter of a photovoltaicinstallation is therefore operated both in the range of good efficiencywith high power delivery and over long periods in the range of poorefficiency with little power delivery. There is therefore a requirementfor an inverter which can be operated effectively even at low powerdelivery.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method foroperating an inverter, which overcomes the hereinafore-mentioneddisadvantages of the heretofore-known methods of this general type andwith which the inverter is effectively operated even in a range of lowpower delivery.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for operating an inverter forconverting direct current into alternating current. The method comprisesproviding direct-voltage terminals, alternating-voltage terminals and aplurality of power switching elements clocked at high-frequency andconnected between the direct-voltage terminals and thealternating-voltage terminals. The high-frequency clocking of the powerswitching elements of the inverter is switched off around a zerotransition (zero crossing) of the alternating current for a period thatdepends on a direct voltage present at the direct-voltage terminals ofthe inverter and/or an output power of the inverter.

By switching off the high-frequency clocking of the power switchingelements of the inverter around a zero transition of the alternatingcurrent for a period which depends on the direct voltage present at thedirect-voltage terminals of the inverter and/or the output power of theinverter, the operating range of the inverter with little efficiency isblanked out and the inverter is mainly operated in the operating rangewith good efficiency.

In this context, the invention is based on the following considerations.During high-frequency clocking of the power switching elements, areactive current which causes a power consumption in correspondingcircuit components usually flows in conventional inverters. The powerpresent at the alternating-voltage terminals or fed into a system isproportional to the current. With a low current in the vicinity of thezero transitions, the transmitted power, due to the high-frequencyclocking, is low in comparison with the above-mentioned powerconsumption so that the efficiency is poor in this operating range. Withincreasing current, this efficiency becomes correspondingly better. Themethod according to the invention blanks out these ranges with poorefficiency and the power is mainly transmitted in the range having agood efficiency so that the total achieved efficiency of the inverter isalso improved.

In accordance with another mode of the invention, the period duringwhich the high-frequency clocking of the power switching elements isswitched off around a zero transition is selected to be all the longerthe lower the output power of the inverter is in the partial-load rangeof the inverter. In other words, a range with poor efficiency is blankedout which is all the greater the lower the current generated by theinverter or the output power of the inverter is.

In accordance with a further mode of the invention, the power switchingelements are activated in the partial-load range of the inverter in sucha manner that the current generated at the alternating-voltage terminalschanges with the slope of the curve of the voltage present at thealternating-voltage terminals. As an alternative, the power switchingelements can also be activated in such a manner that the currentgenerated at the alternating-voltage terminals changes with the slope ofthe square of the curve of the voltage present at thealternating-voltage terminals.

In accordance with an added mode of the invention, the high-frequencyclocking of the power switching elements of the inverter is switched offaround a zero transition of the alternating current with equally longtime intervals for rising and falling edges. As an alternative, thehigh-frequency clocking of the power switching elements of the invertercan also be switched off around a zero transition of the alternatingcurrent with differently long time intervals for rising and fallingedges.

In accordance with an additional mode of the invention, in the peak-loadrange of the inverter, the period during which the high-frequencyclocking of the power switching elements is switched off around a zerotransition is preferably selected to be zero or minimal.

In accordance with yet another mode of the invention, since, dependingon the dimensioning of the circuit components of the inverter, itsefficiency decreases again from a certain output power, the powerswitching elements are preferably activated in the peak-load range ofthe inverter in such a manner that the current generated at thealternating-voltage terminals has the shape of a flattened sinusoidalcurve.

In accordance with yet a further mode of the invention, the frequency ofthe high-frequency clocking of the power switching elements of theinverter is matched to a root-mean-square value of the current generatedat the alternating-voltage terminals.

In accordance with yet an added mode of the invention, the powerswitching elements of the inverter are activated in such a manner thatthe current generated at the alternating-voltage terminals has a curveshape with few harmonics.

In accordance with yet an additional mode of the invention, the methodof the invention described above is of advantage, in particular, whenused for a transformerless inverter. In principle, however, the methodcan also be used with inverters having a transformer. However, due tothe power losses caused by the transformer, it is not as effective inthis case as with a transformerless inverter.

In accordance with again another mode of the invention, the method ofthe invention described above is also of advantage when used for aninverter for feeding current into an alternating-voltage system.However, the method can also be used similarly with an inverter forsupplying power directly to loads.

In accordance with again a further mode of the invention, the method ofthe invention described above is preferably used for an inverter, at thedirect-voltage terminals of which a photovoltaic direct-voltage source,a fuel cell, a battery or the like is connected.

In accordance with a concomitant mode of the invention, in principle,the method of the invention described above can be advantageously usedfor all types of inverters, particularly also for inverters in which thepower switching elements contain half- or full-bridges, reverseconverters, boost choppers, buck choppers, Cuk converters or Sepicconverters or the like.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method for operating an inverter, it is nevertheless not intendedto be limited to the details shown, since various modifications andstructural changes may be made therein without departing from the spiritof the invention and within the scope and range of equivalents of theclaims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic diagram of a circuit topology of abridge-connected single-phase transformerless inverter in which themethod of the invention can be applied;

FIG. 2 includes two diagrams showing variations with time of alternatingvoltage and alternating current in an ideal case;

FIG. 3 is a diagram showing variations with time of alternating currentsfor three different output powers of the inverter according to a firstillustrative embodiment of the invention;

FIG. 4 is a diagram showing variations with time of alternating currentsfor three different output powers of the inverter according to a secondillustrative embodiment of the invention; and

FIG. 5 is a diagram showing a variation with time of an alternatingcurrent in the case of the peak-load range of the inverter according toa further illustrative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the text which follows, the invention will be explained by using theexample of a single-phase transformerless inverter for transformingelectrical direct voltage of a direct-voltage source into alternatingvoltage for feeding energy into an alternating-voltage system of thesame phase and without a phase shift between voltage and current.

However, the present invention is not restricted to this applicationonly. The method according to the invention is similarly suitable forfeeding into systems with 50 Hz, 60 Hz or 400 Hz with single-phase andmulti-phase inverters, with transformerless inverters and inverters withtransformers. The inverter also does not necessarily need to be used forfeeding energy into an alternating-voltage system, rather the methodaccording to the invention can also be advantageously used for inverterswhich are a component of an alternating-current source for directlysupplying loads.

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a known circuit topologyof a bridge-connected single-phase transformerless inverter. Adirect-voltage source 1 such as, for example, a photovoltaicdirect-voltage source, a fuel cell, a battery or the like, is connectedat two direct-voltage terminals 1 a and 1 b of the inverter. Theinverter converts the direct voltage present at the direct-voltageterminals 1 a, 1 b into an alternating voltage and feeds electricalenergy through two alternating-voltage terminals 13 a and 13 b into analternating-current system with, for example, 50 Hz, 60 Hz or 400 Hz ofthe same phase and a current without a phase shift.

The inverter contains a buffer capacitor 2 connected in parallel withthe direct-voltage source 1 and a bridge circuit formed of four powerswitching elements 3 to 6. The power switching elements 3 to 6 areconstructed as high-frequency switches which are suitable for switchingprocesses up to a few 100 kHz. Freewheeling diodes 7 to 10, which areeach connected in parallel with a respective one of the power switchingelements 3 to 6, accept the current during commutation phases of thepower switching elements 3 to 6. The alternating-voltage terminals 13 a,13 b are each associated with a respective choke 11 and 12. The chokes11, 12 provide for a sinusoidal current with little ripple at thealternating-voltage terminals 13 a, 13 b. For better clarity, furthercomponents such as, for example, filters for improving electromagneticcompatibility, have been omitted in FIG. 1.

In order to convert the direct voltage present at the direct-voltageterminals 1 a, 1 b into an alternating current to be fed into a systemthrough the alternating-voltage terminals 13 a, 13 b, the powerswitching elements 3 to 6 are opened and closed in a mutually matchedmanner with certain high-frequency clock patterns. As a result, voltagepulses which can be distinguished from one another discretely in timeare generated, and the potential level thereof is matched to the systemvoltage. During a positive half wave, for example, the two powerswitching elements 3 and 6 are activated with a high-frequency clock anddifferent pulse widths and during a negative half wave it is the powerswitching elements 4 and 5 which are activated. In order to generate,control and regulate the variation with time of the high-frequencyclocking of the power switching elements 3 to 6, it is possible to useboth hardware circuits and software with digital signal processors.

The method according to the invention for operating such an inverter (asan example of an inverter) will now be described in detail through theuse of various illustrative embodiments, referring to FIGS. 2 to 5.

FIG. 2 shows that a current i generated at the alternating-voltageterminals 13 a, 13 b of the inverter (curve 21) and a voltage U presentthere (curve 20) are almost sinusoidal and in phase with one another inthe ideal case. If the inverter is connected for feeding electricalenergy into an alternating-current system, the voltage U present at thealternating-voltage terminals 13 a, 13 b corresponds to the systemvoltage.

FIG. 3 shows the current variation at the output of the inverteraccording to the method of the invention for three different outputpowers in the partial-load range. In the case of a very low power fedinto the system, the current i only flows in the range of the maximumsinusoidal voltage of the system (curve 32). Before and after the zerotransitions of the system voltage, the high-frequency clocking for thefour power switching elements 3 to 6 is respectively switched off orsuppressed. The chokes 11, 12 of the alternating-voltage outputs 13 a,13 b are thus decoupled from the direct-voltage source 1 and the buffercapacitor 2. No current flows either through the chokes 11, 12 orthrough the power switching elements 3 to 6, i.e. the power section ofthe inverter does not consume any power in this operating range in whichthe inverter actually has a low efficiency, as described above. When theclocking is switched on in the range of the maximum sinusoidal voltage,the current then flows with an acceptable efficiency of the inverter.

With a mean power fed into the system by the direct-voltage source 1,the current i flows with a good efficiency over a much greater period(curve 31). In the case of the current curve 30, for an even higheroutput power of the inverter, the power switching elements 3 to 6 arenow interrupted for only a short time around the zero transition of thesystem voltage.

The power which is respectively fed into the system or generated at thealternating-voltage terminals 13 a, 13 b, is proportional to the currenti which is respectively fed in or generated. With a low current in thevicinity of the zero transitions of the sinusoidal wave, theinstantaneous efficiency of the inverter is poor since the transmittedpower is low in comparison with the losses due to the high-frequencyclocking in the power switching elements 3 to 6 and the chokes 11, 12.This particularly applies in the partial-load range of the inverter.According to the invention, therefore, the high-frequency clocking ofthe inverter with its losses is suppressed or switched off for a shorteror longer time around the zero transitions of the alternating voltage Uas a function of the respective current or of the output power of theinverter so that the range of poor efficiency is blanked out.

The current i generated at the alternating-voltage terminals 13 a, 13 bis therefore no longer sinusoidal as shown in FIG. 3. However, thesystem half wave in the center of a system half wave is correspondinglylarger in order to transmit the same total power. Due to the betterefficiency of the inverter in this range, an output power of theinverter which is higher overall is thus achieved.

The method for operating an inverter illustrated in FIG. 3 also achievesa good efficiency in the partial-load range. The hard starting andstopping of the current i according to the curves 30 to 32 during thesystem half waves, however, creates harmonics which are unwanted.

FIG. 4 therefore shows a further illustrative embodiment of a method foroperating an inverter. The method operates in accordance with the samebasic principle as the method described above. i.e. the high-frequencyclocking of the inverter is switched off or suppressed for a shorter orlonger time around the zero transitions of the alternating voltage as afunction of the respective current or of the output power of theinverter in order to blank out the range of poor efficiency.

In contrast to the first illustrative embodiment, however, currentcurves 40, 41, 42 follow the slope of the alternating voltage in therange of starting and stopping. This current shape can be implementedwith simple circuits and operates much more advantageously with respectto the generation of harmonics. The power switching elements 3 to 6could also be optionally activated in such a manner that the currentgenerated at the alternating-voltage terminals 13 a, 13 b changes withthe slope of the square of the curve of the voltage present at thealternating-voltage terminals.

The two illustrative embodiments explained with reference to FIGS. 3 and4 in each case relate to the operation of an inverter in itspartial-load range. Depending on the dimensioning of the power switchingelements 3 to 6 and of the chokes 11, 12, however, the efficiency of theinverter drops again from a certain transmitted power.

In order to achieve a good efficiency even with a peak load of theinverter, a current curve shape 50 illustrated in FIG. 5 is proposed.Due to the rapidly rising current i after the zero transition of thealternating voltage, good instantaneous values are quickly achieved forthe efficiency of the inverter in the peak-load range. The period duringwhich the high-frequency clocking of the power switching elements 3 to 6is switched off around a zero transition of the alternating voltage istherefore preferably selected to be minimal or as zero in the peak-loadrange of the inverter.

In the remaining range, the peak value of the current i remains belowthe sinusoidal curve for the same transmitted power. In other words, thehigh-frequency clocking of the power switching elements 3 to 6 iscontrolled in the peak-load range of the inverter in such a manner thata current curve 50 in the form of a flattened sinusoidal curve isobtained. In this manner, the instantaneous efficiency is also improvedduring the peak values of the alternating voltage.

The method for operating an inverter described above is particularlyadvantageously applicable with an inverter for feeding energy generatedby a photovoltaic direct-voltage source into an alternating-currentsystem without it being intended for the present invention to berestricted to this special application.

1. A method of operating an electrical inverter for converting directvoltage into alternating voltage, the method which comprises thefollowing steps: providing direct-voltage terminals, alternating-voltageterminals and a plurality of power switching elements clocked athigh-frequency and connected between the direct-voltage terminals andthe alternating-voltage terminals; and switching off the high-frequencyclocking of the power switching elements at a zero transition of thealternating current for a period depending on at least one of a directvoltage present at the direct-voltage terminals of the inverter or anaverage output power of the inverter.
 2. The method according to claim1, which comprises switching off the high-frequency clocking at eachzero transition of the alternating current.
 3. The method according toclaim 1, which further comprises, in a partial-load range of theinverter, selecting the period, during which the high-frequency clockingof the power switching elements is switched off at the zero transitionof the alternating current, to become longer as the output power of theinverter becomes lower.
 4. The method according to claim 1, whichfurther comprises, in a partial-load range of the inverter, activatingthe power switching elements to cause a current generated at thealternating-voltage terminals to change with a slope of a curve of avoltage present at the alternating-voltage terminals.
 5. The methodaccording to claim 1, which further comprises, in a partial-load rangeof the inverter, activating the power switching elements to cause acurrent generated at the alternating-voltage terminals to change with aslope of a square of a curve of a voltage present at thealternating-voltage terminals.
 6. The method according to claim 1, whichfurther comprises switching off the high-frequency clocking of the powerswitching elements of the inverter at the zero transition of thealternating current with equally long time intervals for rising andfalling edges thereof.
 7. The method according to claim 1, which furthercomprises switching off the high-frequency clocking of the powerswitching elements of the inverter at the zero transition of thealternating current with differently long time intervals for rising andfalling edges thereof.
 8. The method according to claim 1, which furthercomprises, in a peak-load range of the inverter, selecting the period,during which the high-frequency clocking of the power switching elementsis switched off at the zero transition of the alternating current to beminimal or zero.
 9. The method according to claim 1, which furthercomprises, in a peak-load range of the inverter, activating the powerswitching elements to cause a current generated at thealternating-voltage terminals to have a shape of a flattened sinusoidalcurve.
 10. The method according to claim 1, which further comprisesmatching the frequency of the high-frequency clocking of the powerswitching elements of the inverter to a root-mean-square value of acurrent generated at the alternating-voltage terminals.
 11. The methodaccording to claim 1, which further comprises activating the powerswitching elements of the inverter to cause a current generated at thealternating-voltage terminals to have a curve shape with few harmonics.12. The method according to claim 1, wherein the inverter is atransformerless inverter.
 13. The method according to claim 1, whereinthe inverter feeds current into an alternating-voltage system.
 14. Themethod according to claim 1, which further comprises connecting aphotovoltaic direct-voltage source, a fuel cell or a battery at thedirect-voltage terminals.
 15. The method according to claim 1, whereinthe power switching elements contain half-bridges or full-bridges,reverse converters, boost choppers, buck choppers, Cuk converters orSepic converters.